Lower Ball Joint Issue

[lkt1954]

I'm in the process of installing a coil over conversion from POL on a 69 chevy truck (bare fame). I have the upper and lower control arms and coil overs installed on both sides.
When I go to put the spindle the right side the castle nut will not screw very far down on the lower ball joint stud, it will only screw down to where the castles start- then it binds up. I tried the castle nut from the other side same thing, yet both castle nuts screw on the other lower ball joint stud just fine.
I thought maybe a damaged thread, so I took a 5/8"-18NF die and ran it all the way down the stud to clean the threads, a little resistance but not much. The second time I ran the die on the stud I could do it by hand till it bottomed out. Tried the castle nut again and the results were the same it will go as far as where the castle start and no further. I didn't try to force for fear of galling the nut to the stud.
It's almost like the shaft is a little larger and the nut stop there, yet I can run a die down it with no problem.
The only thing I have not done is run a tap through the nut and I haven't because it goes on the other side shaft just fine.
Any ideas of what could be going on here?
Thanks